Panic with FlexibleContexts and rewrite rules

I wanted a rewrite rule that fired if one of the type variables was an instance of Ord. To make that work, I tried something like this:

{-# LANGUAGE FlexibleContexts #-}

{-# RULES "suspicious" forall (x :: a) y. f (x :: Ord a => a) y = g x y #-}

{-# NOINLINE f #-}
f :: a -> a -> Bool
f x y = True

g :: Ord a => a -> a -> Bool
g = (<)

main = print $ f 2 1

Which generates a rewrite rule like this:

"suspicious" ALWAYS
    forall {@ a $dOrd :: Ord a x :: a y :: a}
      f @ a x y
      = g @ a $dOrd x y

Naturally, $dOrd isn't present on the left hand side so we get a panic.

Is there any way to write this kind of rule without inducing a panic? I suspect that there isn't, but would like to be proven wrong.. sometimes you can optimise better if you can e.g. see that a type is Orderable.

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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